This invention relates to enhancing the stability of roadbeds and particularly to the use of potassium formate, with or without a polymeric viscosifying agent, to enhance the stability of material forming a base or sub-base for road and highway surfaces.
Preparation for road paving generally includes compaction of the base or sub-base, which may comprise clay, gravel, crushed stone, and the like, either taken from the native materials or transported to the site. Frequently, the material includes crushed or otherwise particulated concrete and/or asphalt from the old roadway. Whether the material is primarily reclaimed from an old roadway surface material, taken from a new or old base on site, or is made from materials transported to the site, it is commonly tested for stability. The tested stability of a given mix of materials will be used as an important criterion in determining the thickness of the new pavement to be laid for a road having an expected type of traffic or load. Generally, a road or highway expected to have a great deal of heavy usage will require more concrete or asphalt than one built for relatively light or less frequent use, but an unstable base, in either case, can result in rapid damage to the pavement.
In preparing a roadbed from the materials at hand, or from imported materials, or from a mixture of them, the highway engineer may consider the bed material""s permeability, elasticity, plasticity, cohesion, shearing strength, compressibilty, shrinkage and swell, and frost susceptibility, among other properties. Each of these properties is well known in highway engineering and may be considered an important factor in the choice of the bed mix or any additives for it. This invention is concerned with shrinking and swelling, which is primarily a property of clays.
A succinct statement of the problem addressed by this invention is found in xe2x80x9cSoils Manual for the Design of Asphalt Pavement Structures,xe2x80x9d No. 10 of the Asphalt Institute Manual Series, March 1978 edition, page 10:
xe2x80x9cShrinking and swelling are more pronounced in the fine grained soils, especially clays. Both shrinking and swelling result from a build-up and release of capillary tensile stresses within the soil""s pore water and the varying degree of thirst for water that certain clay minerals have. Fortunately, most high volume change soils in the United States occur in belts or regions that are well known to most soil engineers. In most highway situations, the use of these high volume change soils should be avoided if possible. Where their use cannot be avoided, measures often are taken to reduce their capacity to swell, or to reduce fluctuations in their moisture content. Clays with high volume change capacity frequently have high liquid limits and plasticity indices.
Many roads built to last during the Roman Empire followed the above advice to avoid the use of clay, employing xe2x80x9cflagstone foundations covered with successive layers of rubble and concrete, topped with more flagstones, rammed-down gravel, or concrete. Like modem roads, they were convex, allowing easy drainage.xe2x80x9d Ancient Inventions, by Peter James and Nick Thorpe, Ballantine Books, New York, 1994 p. 52. More recently, various solid additives have been proposed to make road bed aggregates or to add to the available materials to alter the base mixture. See Leonard et al U.S. Pat. No. 4,106,296 (epoxy resins), Jones et al U.S. Pat. No. 4,373,958 (lime kiln dust), Roberts et al U.S. Pat. No. 5,820,302 (silicate and a small amount of cement) and Weber U.S. Pat. No. 5,766,338 (fly ash). However, the modem highway engineer frequently has no choice but to use the clay-containing material most readily available to him.
The density of the compacted base material is an important factor in stability; but it is also known that different compacted materials and mixtures have different rates of distortion or slippage under pressure, and this has come to be an additional important factor in balancing the economics of pavement thickness against longevity. Shrinkage, swelling, and flow rates are formidable factors to contend with when it is necessary to use clay as a significant portion of a road base material.
In recent years it has become common to treat particulate base material with lime or solutions of calcium chloride to improve its stability. However, calcium chloride is not environmentally benign; rather, the widespread addition of large quantities of chlorides to the earth is environmentally questionable. A more acceptable method of roadbed consolidation is needed.
A 1973 patent to Arora, U.S. Pat. No. 3,732,698, proposes a wide range of compounds for use in soil stabilization, and potassium formate is listed along with a large number of other compounds. See column 1, line 49, 56 to column 2 line 19. But potassium formate was not tested with any type of soil, notably not with clay, and the authors submit the following admonition, at column 5, line 64: xe2x80x9cFrom the above experiments it is apparent that no one single agent is best suited for any particular soil. In some cases, a fluoride salt such as potassium fluoride may be particularly effective such as with a Danville type soil. In other cases it may be particularly ineffective such as with a soil from Vancouver, Wash. Thus, the need for testing samples of a given soil which may include a chemical analysis of its pore water before a treatment program is begun is evident. It is also apparent that certain phosphate salts and most of the dicarboxylic acid salts are effective for almost all the types of soil tested.xe2x80x9d Thus no basis is given for using potassium formate in any kind of base material, and certainly no suggestion is made as to its effect on clay. Similar indiscriminate listings and groups are proposed by Kuhn in U.S. Pat. No. 3,490,241 (see particularly col. 1, lines 64-68) and Graf in U.S. Pat. No. 3,959,975, suggesting xe2x80x9croutine laboratory proceduresxe2x80x9d (col. 3, lines 2-3) to choose an appropriate treating agent.
Water soluble polymers have also been suggested in soil stabilization contexts. See Eilers U.S. Pat. No. 3,772,893, Goodhue et al U.S. Pat. No. 6,248,697, and Cargle, U.S. Pat. No. 4,592,931.
I have invented a new method of treating clay-containing roadbed materials to enhance their stability. The enhanced stability enables the highway specification writer or the highway engineer or contractor to predict longer life for the road surface and/or to economize on its thickness. My invention includes the addition of potassium formate, preferably in the form of an aqueous solution, to the base material containing at least 2%, up to 100%, clay. Any clay may be used. As the data below show, a stable material results. I may include with my potassium formate solution a small amount of a viscosifying polymer such as polyacrylamide, polydimethyldiallyl ammonium chloride, or any other water-soluble polymer which is useful and, preferably, environmentally acceptable. I prefer to use a cationic polymer.